Flexible sand drain formed in a casing

ABSTRACT

A flexible sand drain formed in a casing driven into so poor a subsoil that holes cannot be formed therein by an auger comprises a sand pillar, a protector for holding the sand pillar, and a pair of lugs disposed diametrically opposite each other and extending outward from the outer periphery of the protector along the full length of the protector. The protector is made of a plain woven fabric formed by interlacing, at right angles with each other, warps and wefts constituted of monofilaments having a fineness of 300 to 500 denier, so as to leave interfilament meshes large enough to allow sand to be flowed out therethrough. When packed with sand, the protector has a diameter of 5 to 15 cm. The protector has a lengthwise tensile strength of 80 to 120 kg and crosswise tensile strength of 60 to 80 kg per 5 cm width of the protector as measured when it is flattened. The lugs serve to prevent the protector from being twisted when the protector is lowered into the casing. During the withdrawal of the casing the protector packed with sand is prevented from being broken or pulled back by the casing since sand passed through the interfilament meshes of the protector is accumulated in a clearance defined between the casing and the protector to serve as a lubricant. As a result, a sand drain is assuredly formed down to a predetermined depth. The sand drain can be sufficiently flexed according to the movement of the surrounding poor subsoil caused by external forces.

United States Patent 191 Terashima et al.

[ Dec. 30, 1975 [54] FLEXIBLE SAND DRAIN FORMED IN A CASING [75] Inventors: Osamu Terashima, Tokyo; Ikuo Okabayashi, Yamato, both of Japan [73] Assignee: Chiyoda Chemical Engineering and Construction Co., Ltd., Yokohama, Japan 22 Filed: Dec. 3, 1974 21 Appl. No.: 529,231

Related U.S. Application Data [63] Continuation-impart of Ser. No. 381,587, July 23,

1973, Pat. No. 3,859,798.

Primary Examiner.lacob Shapiro Attorney, Agent, or Firml'lolman & Stern [57 ABSTRACT A flexible sand drain formed in a casing driven into so poor a subsoil that holes cannot be formed therein by an auger comprises a sand pillar, a protector for holding the sand pillar, and a pair of lugs disposed diametrically opposite each other and extending outward from the outer periphery of the protector along the full length of the protector. The protector is made of a plain woven fabric formed by interlacing, at right angles with each other, warps and wefts constituted of monofilaments having a fineness of 300 to 500 denier, so as to leave interfilament meshes large enough to allow sand to be flowed out therethrough. When packed with sand, the protector has a diameter of 5 to 15 cm. The protector has a lengthwise tensile strength of 80 to 120 kg and crosswise tensile strength of 60 to 80 kg per 5 cm width of the protector as measured when it is flattened. The lugs serve to prevent the protector from being twisted when the protector is lowered into the casing. During the withdrawal of the casing the protector packed with sand is prevented from being broken or pulled back by the casing since sand passed through the interfilame'ht meshes of the protector is accumulated in a clearance defined between the casing and the protector to serve as a lubricant. As a result, a sand drain is assuredly formed down to a predetermined depth. The sand drain can be sufficiently flexed according to the movement of the surrounding poor subsoil caused by external forces.

5 Claims, 9 Drawing Figures U.S. Patent Dec. 30, 1975 Sheet 1 of2 3,928,979

FIG.

FIG. 2

FIG. 3

FIG. 4

i TIME ELAPSED (hr) 5 1 .1 0 30h: v QMZZKQ Kmhq Sheet 2 of 2 US. Patent Dec. 30, 1975 E mm m 5 m 9. w 2. -91 wE m 0;

"CROSS-REFERENCE TO THE, RELATED ,APPLlCATlON This is a continuation in-partapplicationof the US;

Pat. application Scr. No. 381,587, filediJuly 23, 1973,.

nowU. S Pat. No. 3,859,798, issuedjan. l4, 1975';

.BACKGROUND-OFTHEINVENTION l. Field of the Invention t r This invention relates to a flexible sand drain for improving a poor subsoil, particularly a reclaimed soft ground or an extremely poor subsoilformed of a cohesive soil li'qiiified or semi-liquified due to a large amount of water contained therein. i v

2. Description of the'Prio'r'Art Generally, a poor subsoil to,which asand drain is applied comprises acohesivejsoil including silt, clay and colloid. According to the Standard of International Society of Soil Mechanics Foundation Engineering, the particle sizes of the silt,, clay andcolloid are 'spe cified to be 0.074 to 0.005 mm, 0,.005t 0.001 mm and 0.001 mm or less, respectively.

With the natural moisture content and liquid limit of 2 the cohesive soil represented by Wn and L.L.,resp'ec tivcly, such a high moistureor water-bearing subsoilas approximately expressed by i i v Wn 2 0.8 L.L.

that holes cannot be formed by an-auger in sucha poor subsoil. i i v A sand drain consisting of only a sand pillar has been conventionally used in improving a'poor subsoil. However, when a load is applied on the sanddrain after the installation of the sand drain in an extremely poor sub'- soil, the sand pillar constituting the sand drain cannot be deformed according to the mbvementofthe-subsoil since the allowable deformation of the sand pillar is far smaller than that of the cohesive soil, and the sand drain is eventually pinched off, whereby the integrity or continuity of the sand drain'is broken. Thus, the sand drain fails to perform a water-draining function.

In an attempt to prevent the breakage of the sand drain, a sand drain, hereinafter referred to as a patented sand drain", contained within a protector is disclosed in U.S. Pat. No. 3,396,541. I

The protector of the patented'sand drain installed in the subsoil' has such a mechanical strength that it remains upright even when the sand pillar is subjected to relatively greater external forces such as shearing stresses due to a cohesive soil, vertical compression stresses, a pore water pressure, etc. A rei nforcing meansis also used to assure the mechanical strength of the protector. Namely, the sand drain'resists any exterintothe sand drain, those small, particles of the cohesive soil which are included in, or vcarried with, the waterare deposited on the outer surface of the -protec tor to form a water-impermeable film. As a result,'the waterpermeability of the protector is abruptly reduced withthe consequent disadvantage.

SUMMARY OF THEINVENTION An object of this invention is to provide a flexible sand drain which is capable of beingb'ent according to the movement of the cohesive soil occurring in the surrounding subsoil, so as to prevcntvthe sand pillar from being pinched off or, broken.

Another object of this invention is to provide a flexible sand drain suitable for beingfor'med'within a casing which is driven into a poor su.bsoil. 'Anotherobject of this invention is to p'rovide'a flexible sand drain performing an effieient and sufficient water-draining function without reducing its waterdraining capability with the passage bf time.

Another object of this invention is to provide a sand drain which is low 'ininstallation cost and requires fewer installing steps. I I

Another object of this invention is to provide a sand drain whichconcurrently satisfies all of the above-mentioned objects of this invention. I i

According to this invention there is provideda sand drain comprisinga sand pillar formed by sand, a tubuis generally called an extremely poor subsoil.'lt is noted. lat protector for hlding the said pillar and a pair of lugs formed on the outer peripheral surface of the protector so as to, be diametrically opposed to each other.

nal forces, such as shearing stresses, vertical compression stresses and pore water pressure to which the sand pillaris subjected, and maintains its vertical configuration. However, the patented sand drain is very expensive due to high mechanical strength being required for its protector. p p

The protector of the patented sand drain'is made of woven fabric having very fine meshesor very small openings so as ,t0 preyent a cohesivesoil around the protector from coming into the sand pillar contained within the protector andjpermit only water'in the-surrounding soil to flow into the sand pillar. During the process in which the'water in the surrounding soil flows The lugs extend: outwardifrom the outer peripheral surface of the protector, and along the full length 3 5 thereof such that they prevent the protector from being 40 ,pellent, anti-weathering, chemically non-corrosive 'monofilaments so interlaced as todefine meshes or openings large enough to allow the sa'ndto flow out of theprotector-..

The warps intersect with the wefts at right angles and '45 extend lengthwise of the protector,. thereby ,reducing the elongation of the protector toa minimum; when the protector is packedwith sand. I I

The protector has an elongation of less than 5% or so, and it is sufficient for the protector to have the low '50 lengthwise tensile strength of 80 to 120 kg or so and crosswise tensilestrength' of 60 to 80 kgor so per 5 centimeterwidth of the protectoras measured when the protector is flattened.- The protector ismade by interlacing monofilaments .of fineness of about 300 to 500' deniers. 'The"interfilament-meshes ,or-bpenings have an area of the order of 2.0 to 3.0 mm left therebetween. Furthermore, sand constituting the sand pillar flows out of the protector through the meshes or openings as specified above so'that the meshes of the protecv tor are prevented from'being clogged bythe cohesive soilwith the passage of time. BRIEF pEsc iPrio pF "rue- RAWING l is a perspective view of a tubular protector used with a flexible sand' drain according to this inven- FIG. 3 is a transverse cross-sectional view of the protector of FIG. 1;

FIG. 4 is a graph showing the water-draining characteristics of the sand drain according to this invention;

FIGS. 5 through 7 illustrate the method for installing the sand drain in a poor subsoil;

FIG. 8 shows the sand drain installed in a poor subsoil; and

FIG. 9 illustrates the sand drain of'FIG. 8 which is bent due to the movement of the surrounding poor subsoil.

DESCRIPTION OF THE PREFERRED EMBODIMENT Throughout the Figures the same parts are denoted by the same numerals.

A sand drain 10 according to this invention comprises a tubular protector 11 and a sand pillar 12 formed ofa sand 13 filled therein (FIG. 8). The protector 11 is made of plain woven fabric 14 which comprises warps 15 and wefts 16 roughly interlaced at right angles with each other so as to leave interfilament meshes or openings 17 large enough to allow sand 13 to flow out therethrough. The warps l5 and wefts 16 are of water-repellent, anti-weathering, chemically noncorrosive monofilaments 18. The protector 11 is formed, as shown in FIG. 1, by superposing one over the other two narrow strips 19 and 20 obtained by cutting along the warps 15 of the fabric 14 and joining both the strips 19 and 20 at lateral side edge portions 21 defined between the edges 22 of the strips 19, 20 and the corresponding lines 23, hereinafter referred to as joints. The width of the edge portion 21 is several centimeters. However, the exact dimension is decided by the diameter of the protector. The two strips 19 and 20 are joined at their edge portions 21 by thermal fusion, sewing or interlacing along the warps 15. The joined lateral side edge portions 21 form a-pair of lugs 24 extending along the full length of the protector 11. A plurality of protectors '11 can be simultaneously formed by superposing one over the other two sheets of fabric 14, constituted of warps l5 and wefts 16 of monofilaments, with those portions corresponding to the lugs 24 of the protector being joined'by'thermal fusion or sewing and then cutting the fabric 14 lengthwise along the center line of the joined portions thereof. Alternatively, a plurality of protectors 11 can also be simultaneously formed by separately plainweaving those portions of front and back fabric 14 corresponding to main bodies 25 of the protector l1, interlacing those portions of the front fabric with those of the back fabric corresponding to theflugs 24 of the protector l l, and cutting the interlacedportions of the front and back fabric along the center line thereof. The lugs 24 of the so manufactured protectors 11 as abovementioned show increased strength and rigidity as compared with the main body 25 of the protector 11. The number of the warps of the lugs 24 per unit length of the wefts thereof is 2 to 3 times greater than the number of the warps of the main body 25 of the protector 11 per unit length of the wefts thereof. Likewise, the number of the wefts of the lugs 24 per unit length of the warps thereof is 2 to 3 times greater than the number of wefts of the main body 25 of the protector 11 per unit length of the warps thereof. Thus, lugs 24 of the protector 11 exhibit a strength and rigidity sufficient to prevent twisting of theprotector, since they bear almost all the weight of a later-described weight member used in 4 lowering the protector 11 into a casing and render the main body 25 of the protector 11 substantially free from the weight of the weight member.

As will be apparent from the foregoing, the protector 11 comprises the main body'25 defined between the joints 23 and the pair of lugs 24 provided at each side of the outer periphery of the main body 25. When the protector l l is packed with sand 13, it assumes a cylin drical configuration having a diameter D (FIGS. 1 and 3) with the'lugs 24 positioned diametrically opposite each other and extending outward from the peripheral surface of the protector 11 along the full length of the protector 11.

More specifically, the protector 1*] is designed to have the diameter D of 5 to 15 cm which is determined by the length of the protector 11.

Experiments show that, as measured per 5 cm width when flattened, the protector 11 should have the lengthwise tensile strength of about 80 to 120 kg and the crosswise tensile strength of about to 80 kg.

It is essential that the protector 11 according to this invention be constituted of lattice-formed plain woven fabric having parallel warps 15 extending lengthwise of the protector 11 and parallel wefts 16 extending crosswise of the protector 11.

The protector, knit or woven by other processes including twilling, other than the plain woven pattern of this invention, tends to be locally bulged crosswise as the protector is increasingly packed with sand. The bulging not only prevents the uniform formation of the sand drain diameter, but also is a cause for trouble during the withdrawal of the casing to be described later.

In contrast, the sand drain of this invention has its protector formed of plain woven fabric bearing the lattice form. Therefore, the lengthwise and crosswise elongations of the fabric give rise to considerably fewer troubles than-the fabric prepared by other weaving or knitting processes. However, the protector made of such latticed fabric is still unavoidably subject to elongation. To restrict the occurrence of this event, therefore, the quality of monofilaments used with the protector should be carefully selected to limit the lengthwise and crosswise elongations of the protector within 5%.

In addition to restrictions on the elongation, the protector l 1 should be made of water repellent, antiweathering, chemically non-corrosive monofilaments 18. In actual practice polyethylene monofilaments are suitable. Nylon and Orlon are generally expensive and it is difficult to restrict their elongation. Vinylon is poor in its water-repellency and rapidly loses its strength upon contact with the subsoil water, with consequent damage. In contrast, the polyethylene monofilament per se is non-water permeable and excellent in its anti-weatherability and its corrosion-resistant properties which are required from the practical viewpoint. Furthermore it retains its strength even when contacted with water and, in consequence, has the'necessary strength. The elongation of the protector 11 made of the polyethylene monofilaments can be restricted to about 5% and the protector l 1 can satisfy the necessary strength. The

, interfilament meshes or openings 17 of the protector l l are 2 to 3 mm, which are large enough toallow part of the sand 13 within the protector 11 to freely pass therethrough. The fineness of the monofilament 18 is within the range of 300 to 500 denier as restricted by the size of the mesh or opening 17 of the protector ll 5 as well as the required strength of the protector ll.

The method for installing the sand drain according to this invention will now be explained below.

In an attempt to install the sand drain in a poor subsoil 34, particularly extremely poor subsoil, a hole bored in the earth is absolutely required to maintain its configuration at least until the sand drain is installed therein. For this reason and for the purpose of providing a desired size of a hole in the subsoil, it is recom mended that a steel casing 26 with a bottom cover 27 is driven into the subsoil by a pile driver (not shown, FIG. 5), and then the protector 11 is lowered as shown in FIG. 6 into the casing 26. Since the inner surface of the casing 26 is smooth and substantially completely circular in cross section, it is much easier to lower the protector 11 into the casing 26than into holes formed by an auger.

The protector 11 according to this invention is flexible or pliable in nature, light in weight and elongated in configuration and difficulty is encountered in smoothly lowering the protector 11 into the casing 26. If, however, a weight member 28 is mounted directly on, or suspended through a hook 29 from, a lower end 11a of the protector 11, the protector 11 can be easily lowered to the lower end of the casing 26. The weight member 28 may be replaced by a small amount of contained sand.

Where use is made of a tubular protector having no lugs 24 or its counterparts, the protector tends to be rotated, due to the load of the weight member 28 being localized while lowering the protector into the casing, causing it to be locally twisted or bent. As a result, the sand charged into the protector 11 is stopped at the bent portion of the protector. 11, thereby preventing the sand 13 from reaching the lower end of the protector 11. Even if the sand 13 reaches the bottom, the sand drain l often fails to assume a complete cylindrical configuration having the desired depth and diameter, since the sand pillar 12 is locally constricted.

The protector 11 according to this invention has the pair of lugs 24 facing diametrically opposite each other and extending outward from the outer periphery thereof. Furthermore, the lug 24 of the protector 11 is more rigid than the main body 25 thereof. This arrangement enables the protector 11 to be smoothly lowered to the lower end of the casing 26 as shown in FIG. 6 without local twisting or bending of the protector 11 due to the localized load of the weight member 28. After the protector 11 is lowered to the lower end of the casing 26, the sand 13 is charged through a sand hopper 30 into the protector 11 to form a cylindrical sand pillar 12, as shown in FIG. 7, having the desired depth and diameter. Since the lugs 24 of the protector 11 are more rigid than the main body 25 thereof and are facing diametrically opposite each other and extend outward from the outer periphery of the protector 11 along the full length thereof, they bear almost all the weight of the weight member 28, thereby serving to maintain the protector 11 substantially upright. Rotation of the protector 11 is opposed by the weight of the weight member 28 and the rigidity of the lugs 24. Each lug 24 has a width of l to 3 cm dependent upon the diameter of the protector 11.

After the protector 11 is packed with the sand 13, an upper end portion 11b of the protector 11 is folded up and the protector 11 is enclosed within the casing 26. A sealing cover 31 hinged at the upper end 26a of the casing 26 is closed and fastened by means of a latching device 32. Thereafter, compressed air is introduced into the casing 26 through an inlet pipe 33 disposed at the upper end portion of the casing 26. The casing 26 is pulled out of the poor subsoil 34 by drive means (not shown), while being vibrated by a vibro-hammer (not shown). During the withdrawal of the casing 26, the compressed air acts to push the sand drain l0 downward, thereby preventing it from being pushed up by the pressure of the subsoil 34 which is extremely soft.

After the casing 26 is completely withdrawn, the sand drain 10 is positioned within the subsoil 34, as shown in FIG. 8. If any load 35 is placed on the sand drain 10, the sand drain 10 starts to drain water from the surrounding subsoil 34. The diameter D of the sand drain 10 according to this invention is smaller than that of the prior art sand drain. This feature is important'in realizing the object and function of this inventionfWhen the sand 13 is charged through the sand hopper 30 into the protector 11 which is previously lowered in the casing 26 driven into the poor subsoil 34, difficulty is usually experienced technically and economically in allotting the whole weight of the sand 13'to the protector 11 with its upper end 11b connected to the hopper 30. The weight A borne per unitwidth. of the sand drain 10 per unit length thereof can be expressed as follows:

71' ratio of the circumference of the circular protector 11 to its diameter.

From the above equation, it will be understood that the weight a is increased in proportion with the diameter D of the protector 11. Since the sand drain 10 of this invention is small in diameter, the required strength of the protector 11 can be reduced as will be appreciated from the equation. In actual practice, however, there is a limitin reducing the diameter d of the sand drain 10. It is therefore necessary to reduce the weight applied to the protector 11. According to this invention a certain clearance 36 is left between the casing 26 and the protector 11. The sand 13 passes through the meshes or openings 17 of the protector 11 into the clearance 36 as the sand 13 is charged into the protector 11. As a result, part of the weight of the sand 13 can be borne by the inner wall of the casing 26 due to the internal frictional angle of the sand as well as the frictional resistance of the inner wall of the casing 26. The clearance 36 defined between the casing 26 and the protector 11 is preferably about 4 to 6 mm. The sand 13 varies greatly in fluidity dependent upon its moisture content. The opening or mesh 17 of the protector should have such an area as to allow part of the sand to pass therethrough into the clearance 36 between the protector l1 and the casing 26 even in wet condition. The cross-sectional area of the opening or mesh 17 of the protector 11 is 2 to 3 mm, i.e., large enough to allow larger particles of sands 13 to freely clearance 36, the sand drain l meanders within the casing 26 and there is a possibility that the outer periphery of the protector 11 will be contacted by the inner surface of the casing 26. During the withdrawal of the casing 26, therefore, friction occurs between the inner wall of the casing 26 and the outer surface of the protector 11, causing the protector 1 1 to be broken or the sand drain 10 to be pulled back.

Since, according to this invention, the sand 13 is present in the clearance between the casing 26 and the protector 11, it serves as a protecting agent or a lubricant during the withdrawal of the casing 26 from within the subsoil 34. As a result, any breakage of the protector 11 or pull back of the sand drain 10 does not take place during the withdrawal of the casing 26.

From the above it will be understood that, according to this invention, the sand drain can be installed in the casing without any technical problems.

The sand drain 10 of this invention will be flexibly bent according to the movement of the surrounding subsoil 34 as follows:

Firstly, if the sand drain has the length L, the reduction of the diameter of the sand drain constitutes an important factor in imparting flexibility to' the sand drain. The sand drain 10 according to this invention has the diameter D well satisfying such flexibility.

Secondly, a protector with larger meshes or openings formed by woven thin monofllaments exhibits an excellent flexibility when packed with sand, as compared with a protector with smaller meshes or openings formed by woven thick monofilaments. Since the protector 11 according to this invention is formed by roughly plain weaving sufficiently thin monofilaments 18, it well satisfies the second requirement.

Thirdly, tension applied to the projected portion of the sand drain in the subsoil differs from sand contained in a protector exhibiting suitable elongation and having interfilament meshes or openings large enough to allow part of sand to freely pass therethrough as opposed to a sand drain using a protector which exhibits no elongation and has interfilament meshes or openings small enough to prevent the passage of sand therethrough. Namely, in the latter case, a great tensional force is applied to the projected portion of the protector due to the pressure of the extremely poor subsoil. In the former case in accordance with this invention, on

- the other hand, such tensional force applied to the projected portion of the protector is greatly reduced since the sand within the protector is allowed to freely pass therethrough and the protector allows a certain amount of elongation. Since, in the former case, the protector is made smaller in diameter, it is easily moved in all directions across its cross section and thereby'can be flexibly bent according to any external force. From the above it will be understood that the sand drain 10 according to this invention exhibits good flexibility.

Explanation will now be made as to how the sand drain according to this invention maintains its waterdraining function during the period necessary for water drainage.

Water drainage tests were conducted between the protector according to this invention and a protector having sufficiently small meshes only to permit the passage of water therethrough but to prevent the passage of soil particles present in the surrounding subsoil. The results are shown in the graph of FIG. 4, in which T denotes the water drainage of the protector according to this invention and P denotes the water drainage of the prior art protector. In the prior art protector a cohesive soil was deposited onto the outer surface of the protector and a water-impermeable film was formed during a short period of time, thereby rapidly reducing a water-draining capability. With the protector according to this invention a water-impermeable film is substantially never formed and, in consequence, the water-draining capability was little affected. Since, with the sand drain 10 according to this invention, the sand 13 flows out of the outer periphery of the protector 11, the water-draining area becomes larger than the outer periphery of the protector, serving to decrease an amount of cohesive soil flowing into the protector 11. Moreover, some cohesive soil flowing into the protector provides little if any hindrance to the'water drainage of the sand pillar, since adjacent sand particles makes relative movements with each other due to the meandering movement of the sand drain 10.

The sand drain 10 according to this invention can be installed at low cost in the subsoil 34 as will be explained below.

The advantage that the sand drain 10 according to this invention is smaller in diameter than the prior art sand drain is closely related to the enhancement of the flexibility of the sand drain as mentioned above. The other advantages of the sand drain 10 will be understood from the following theoretical consideration:

According to Barrons theory,

where t the time of consolidation (sec) de the effective spacing of sand drains (cm) Ch the coefficient of consolidation Th the Time factor of consolidation U the percent consolidation.

The effective spacing de of the sand drain as shown in the equation (1) has such a characteristic that it is increased in proportion to the diameter d of the sand drain. If the coefficient of consolidation Ch and Time factor of consolidation Th are constant, it will be known that the time of consolidation 1 is increased in proportion to the square of de. Accordingly, where de is decreased, the diameter of the sand drain can be also decreased. From this, it will be understood that sand drains are installed at lower cost and at a narrower spacing. Let it be assumed that both larger and smaller diameter sand drains are installed in the subsoil of the given area. Although more sand drains are required in the latter case than in the former case to provide the desired consolidation rate, the total amount of sand used is substantially less in the latter sand drains as opposed to the former sand drains. If, for example, comparison is made between a sand drain having a normal diameter of 40 cm and a sand drain having a smaller diameter of 12 cm, the latter requires one-seventh the amount of sand as required by the former to attain the desired consolidation rate.

The sand drain 10 according to this invention exhibits this sand-saving feature. Since the sand drain 10 according to thisinvention has a smaller diameter as mentioned above and is capable of substantially alleviating the load applied thereon due to frictional resistance within the casing, the protector 11 does not require high strength material and reinforcing device as employed in the protector of the sand drain disclosed in U.S. Pat. No. 3,396,541. Furthermore, the protector 11 is formed by roughly plain weaving monofilaments having a normal strength. As a result, a material cost can be greatly reduced.

The sand drain 10 according to this invention is formed within the casing 26. This method is, therefore, more reliable and far more efficient in installation than the case where a sand drain is formed by using an auger.

What we claim is:

1. In a flexible sand drain formed in a casing, including a sand pillar and a tubular protector for holding the sand pillar, the improvement wherein a. said protector comprises a plain woven fabric constituted by warps and wefts of a plurality of waterrepellent, antiweathering, chemically noncorrosive monofilaments so intermeshed as to make interfilament meshes large enough to allow sand to freely pass out of the protector;

b. said warps intersect with said wefts at right angles and extend lengthwise of the protector so as to restrict the elongation of the protector;

c. said protector is provided with a pair of lugs having a higher mechanical strength and rigidness than those of the main body of the protector, comprising warps and wefts of said monofilaments and extending in a diametrically opposed relation along the full length thereof to prevent the protector from 10 being twisted when the protector with a weight member fixed tothe lower end of the protector is lowered into the casing driven into a poor subsoil;

d. the number of said warps of said lugs per unit length of the wefts thereof is 2 to 3 times greater than the number of said warps of the main body of said protector per unit length of the wefts thereof;

e. said protector when packed with sand has a diameter of Ste 15 centimeters;

f. said protector has an elongation of less than 5 percent, and a lengthwise tensile strength of to kilograms and crosswise tensile strength of 60 to kilograms per 5 centimeter width of the protector as measured when the protector is flattened;

g. said monofilaments have a fineness of 300 to 500 denier; and

h. the sand drain when installed in the soft ground on be flexed so as to cope with the movement of the poor subsoil.

2. The sanddrain according to claim 1, in which each of said interfilament meshes has an area of 2 to 3 mm 3. The sand drain according to claim 1, in which each of said lugs has a width of l to 3 cm.

4. The sand drain according to claim 1, in which said lugs comprise interlaced lateral. side edge portions of the protector.

5. The sand drain according totclaim 4, in which each of said lugs has a width of l to 3 cm. 

1. In a flexible sand drain formed in a casing, including a sand pillar and a tubular protector for holding the sand pillar, the improvement wherein a. said protector comprises a plain woven fabric constituted by warps and wefts of a plurality of water-repellent, antiweathering, chemically noncorrosive monofilaments so intermeshed as to make interfilament meshes large enough to allow sand to freely pass out of the protector; b. said warps intersect with said wefts at right angles and extend lengthwise of the protector so as to restrict the elongation of the protector; c. said protector is provided with a pair of lugs having a higher mechanical strength and rigidness than those of the main body of the protector, comprising warps and wefts of said monofilaments and extending in a diametrically opposed relation along the full length thereof to prevent the protector from being twisted when the protector with a weight member fixed to the lower end of the protector is lowered into the casing driven into a poor subsoil; d. the number of said warps of said lugs per unit length of the wefts thereof is 2 to 3 times greater than the number of said warps of the main body of said protector per unit length of the wefts thereof; e. said protector when packed with sand has a diameter of 5 to 15 centimeters; f. said protector has an elongation of less than 5 percent, and a lengthwise tensile strength of 80 to 120 kilograms and crosswise tensile strength of 60 to 180 kilograms per 5 centimeter width of the protector as measured when the protector is flattened; g. said monofilaments have a fineness of 300 to 500 denier; and h. the sand drain when installed in the soft ground can be flexed so as to cope with the movement of the poor subsoil.
 2. The sand drain according to claim 1, in which each of said interfilament meshes has an area of 2 to 3 mm2.
 3. The sand drain according to claim 1, in which each of said lugs has a width of 1 to 3 cm.
 4. The sand drain according to claim 1, in which said lugs comprise interlaced lateral side edge portions of the protector.
 5. The sand drain according to claim 4, in which each of said lugs has a width of 1 to 3 cm. 